What Is It Like to Be a Plant?
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Paco Calvo What Is It Like to Be a Plant? Abstract: In this article, I explore the possibility of plant subjective awareness within the conceptual framework of Plant Neurobiology — an emerging discipline that aims to unearth the way plants perceive and act purposefully. I shall argue that plants lack none of the functional structures that are supposedly needed, and so we have no scientific reason to exclude the possibility that they have evolved different structures that underlie their own subjective experiences. Where you tend a rose, my lad, A thistle cannot grow. — Frances Hodgson Burnett (1911), The Secret Garden 1. Introduction Copyright (c) Imprint Academic 2016 In the opening paragraph of his seminal ‘What Is It Like to Be a Bat?’, For personal use only -- not for reproduction Thomas Nagel (1974) warned us that the problem of consciousness didn’t belong to the same category as, for example, the oak tree– hydrocarbon problem. The latter has to do with more or less success- ful episodes of reductive explanation in the history of science. As plant biochemistry and plant cellular and molecular biology continue making headway, we have an increasing understanding of the details of the substrate, of the carbon compounds that an oak tree, so to speak, reduces to. But consciousness is a whole different business; it’s a truly thorny issue. Correspondence: Paco Calvo, MINT Lab, Department of Philosophy, University of Murcia, Spain; EIDYN Research Centre, and Institute of Molecular Plant Sciences, University of Edinburgh, Scotland, UK. Email: [email protected] Journal of Consciousness Studies, 24, No. 9–10, 2017, pp. 205–27 206 P. CALVO Plants can be thorny too, and by this I don’t mean that some of them can bear thorns! This paper is not about botany, but about the very possibility of plant consciousness. The last decade witnessed the birth of a new discipline, Plant Neurobiology (see Calvo, 2016), whose target is plant signalling and adaptive behaviour. The objective, ulti- mately, is to account for plant intelligence beyond the limits of basic plant science under plant biochemistry or plant cellular and molecular biology. Overall, plant neurobiology aims to unearth the way plants perceive and act. It is in this setting that I wish to pose the following question: ‘What is it like to be a plant?’ In my view, the time is ripe to at least cast the problem in a scientifically tractable manner. Nagel didn’t choose bats for no special reason. And neither have I with respect to plants. In his case, the choice of a mammal was moti- vated with an eye to somehow ease resistance. After all, of those who think human animals are not the one and only species capable of reflection, most probably would be thinking of non-human mammals as potential candidates. Bats are mammals and thus, in principle, appear to fit the bill. But, in so far as they are sufficiently awkward as told from our own experiential standpoint, bats also serve Nagel’s ‘intuition pump’ purpose, to borrow Dennett’s (1991) term. Our sensory apparatus and theirs, let us put it, are not that similar; to the point that, somewhat tongue in cheek, Nagel comments: ‘anyone who has spent some time in an enclosed space with an excited bat knows what it is to encounter a fundamentally alien form of life’ (Nagel, 1974, p. 438). Well, bats, however alien they may look, still are mammals. Let me tell you something: if there are aliens around us, Copyright (c) Imprint Academic 2016 these are plants! On the other hand, we may agree with Nagel that consciousness For personal use only -- not for reproduction spreads pretty widely. Plausibly it is not one of those things only mammals do, but ‘widely’ doesn’t mean it happens indiscriminately all over the place. Some forms of life may have it, some others not. In Nagel’s view, ‘It occurs at many levels of animal life, though we cannot be sure of its presence in the simpler organisms’ (ibid., p. 436). One thing is for sure: if plants happen to have conscious experiences in one form or another, then there must be something it is like to be a plant. Some caveats are in order before further ado. Nagel does not specify what he means by ‘simpler organisms’. Plants and animals are both multicellular eukaryotes, and from a phylogenetic point of view none would qualify as ‘simple’. It is possible that he has in mind prokaryotic single-celled life forms. One way or another, his ‘levels of WHAT IS IT LIKE TO BE A PLANT? 207 animal life’ phrasing excludes plants, regardless of their complexity. On the other hand, there are hundreds of thousands of living green plant species. Thus, to draw the parallel more precisely, the proper question to be posed should be: ‘What is it like to be a rose?’ (or a thistle, for that matter!). Of course, were panpsychism to be correct, then all green plants, being so high up the phylogenetic chain, would uncontroversially be minded. But my main thesis does not rest upon the truth of panpsychism, however strong the arguments in its favour may turn out to be. In what follows, I shall ignore subtleties along these lines for ease of exposition, although we should bear in mind that some plant species might well have evolved consciousness, whereas others may have not. Last, a word on terminology. Despite the emphasis on ‘consciousness’, the ensuing discussion is to be understood in a wider context in so far as various aspects of mentality, including subjective experience, awareness, inner representation, central coordination, goal-directedness, memory, self-awareness, and qualia are discussed. These concepts may be clustered separately as qualitative and intentional aspects of mentality, respectively. This article is primarily about qualitative aspects of the experiences of plants, although in order to elaborate on such subjective character, intentional aspects will be incorporated into the discussion as we proceed. Let us first step down the tree of life in our quest for other forms of experiencing the world, and see what invertebrates are up to, in order to pave the way for a better understanding of plant life. 2. Of Insects, Nematodes, and Cnidaria Copyright (c) Imprint Academic 2016 Barron and Klein (2016) have recently tracked consciousness down For personal use only -- not for reproduction the animal phylogenetic tree. According to them, the origin of con- sciousness in its simplest form probably dates back to the Cambrian explosion, a period that brought land vertebrates in between 542 and 488 million years ago. But as the fossil record shows, most of the animal phyla, not just vertebrates, appeared during this, geologically speaking, brief lapse of time. In fact, Barron and Klein move beyond mammals and vertebrates, and consider subjective experience in insects: The brain structures that support subjective experience in vertebrates and insects are very different from each other, but in both cases they are basal to each clade. Hence we propose the origins of subjective experi- ence can be traced to the Cambrian. (ibid., p. 4900) 208 P. CALVO Colonizing the land must have brought about different selective pressures, and it is not unlikely that new predation relations gave a ‘little’ push to evolutionarily more sophisticated solutions to those of pre-Cambrian life forms. Of course, land vertebrates were not the only ones needing to get used to their new environment. Invertebrates also came out of the Cambrian period with their own needs, evolving their own toolkits. But ‘invertebrate’ is a vast category selection. They are estimated to make up some 95% of all animal species found on Earth. We may thus narrow down our search and pick from insects, nema- todes, cnidaria, annelids, arachnids, crustaceans, mollusks, and a large etcetera. Some of these may have evolved a sense of awareness, and others not. Barron and Klein circumscribe their quest to insects. One way or another, a borderline is to be marked. As expected, plants are out of the game even before the ball has started to roll — ‘However, consciousness also gives out somewhere. Plants do not have it. It would be surprising if jellyfish did’ (ibid., p. 4900). As a philosopher of plant neurobiology, I’m used to plants being neglected in the cog- nitive science literature (Calvo, Martin and Symons, 2014; Calvo, Raja and Lee, 2017; Calvo, Baluška and Sims, 2016), but we’ll get to that soon. First, what is it exactly that makes insects good candidates, and jellyfish bad ones?1 It goes without saying that, when it comes to insects and jellyfish, introspective or verbal reports are of little help. Behavioural studies, by contrast, may help, but have problems of their own. For one thing, the less similar a species is with respect to us (non-mammalians?), the more difficult it is to interpret their behaviour. Bluntly, it seems easier Copyright (c) Imprint Academic 2016 to infer that my dog is happy from his tail wagging (although for some surprises, see Quaranta, Siniscalchi amd Vallortigaraemail, 2007) than For personal use only -- not for reproduction to infer an analogous pattern in the case of an insect. In any case, the risk of anthropomorphizing the very way we think of experiments and our interpretations always lurks in the background. We seem to project our preconceptions into the very experimental setting being designed. In a sense we constrain our subjects to act as they are expected to from our point of view (examples of artificial lab settings abound). On the other hand, when we look at the neural correlates of consciousness (Metzinger, 2000), rather than at overt behavioural 1 For contrast with Barron and Klein (2016), see Tye (2000) who argues that although honeybees may well be conscious it is unclear that the same holds when it comes to other insects.